T cells rely on the exquisite sensitivity, specificity, and speed of T cell receptor (TCR) triggering to properly distinguish pathogenic from non-pathogenic peptides. How T cells factor the spatial and temporal dynamics of extracellular peptides into appropriate TCR triggering and cellular activation is not well understood, largely because the field has lacked tools to specifically manipulate independent parameters of ligand presentation to the TCR. In previous work, we have developed the Phytochrome/PIF photoreversible protein-protein interaction module to for micron-level spatial control and second level temporal control of intracellular signaling. In this study, we are adapting this module for photoreversible activation f TCR signaling. By combining this optogenetic system with assays of TCR engagement, TCR triggering, and cellular activation, we will probe the fundamental question of how TCR ligands are decoded. On a whole-cell level, we are investigating the role of spatial and temporal dynamics of ligand presentation for overall T cell activation and polarization (Aim 1). On a molecular level, we are investigating the role of ligand kinetics in triggering the TCR (Aim 2).